1. Field of the Invention
The present invention relates to semiconductor processing technology, and more particularly, to a method of removing photo resist residues in the manufacturing of semiconductor devices.
2. Description of the Related Art
Photo resist, also called resist for short, is a macromolecular compound whose erosion resistance capacity will be changed after lighting. Photo resist is a photosensitive material. In some semiconductor manufacturing processes, a photo resist layer can be formed on a surface of a device (such as a semiconductor wafer), and a patterned photo resist can be formed after lithography. As such, the surface which is not covered by the photo resist is exposed, and the surface that is covered by the photo resist is protected by the photo resist. Some semiconductor processes, such as etching, deposition and ion implantation can be performed on the exposed surface and the remaining photo resist. After one or more semiconductor processes, the remaining photo resist can be removed with a stripping process. Ashing is, for example, a commonly used process for removing a photo resist.
As the feature sizes of semiconductor devices continue to shrink to 65 nm and beyond, an LDD (Lightly Doped Drain) ashing process gains more attention on film loss control, in addition to the photo resist removal capability. For example, in the LDD ashing process, the loss of the thin film includes the loss of Si, SiGe, and silicon nitride offset spacer, silicon nitride spacer thin film, and the like.
Conventional ashing processes use oxygen (O2), plasma. The main problems include Si recess and a film property change, which may lead to issues of LDD litho patterning scrumming, and have difficulty to meet current process requirements.
Thus, a process which uses a gas mixture including hydrogen (H2) and O2 to generate plasma for the ashing process has been developed. Although the process reduces the loss of Si, it cannot eliminate negative effects such as the loss of the silicon nitride offset spacer and the like. Currently, the ratio of H2 in the gas mixture is about 4%, and the ratio of nitrogen (N2) in the gas mixture is about 96%. In the finer process compared with the 45 nm process, higher ratio of H2, such as 10% is required in order to minimize or reduce the loss of Si. Then, the loss of silicon nitride spacer will become more serious.
FIG. 1 shows a diagram of the comparison between the effect of an ashing process using O2 and the effect of an ashing process using a H2/N2 gas mixture. The diagram labeled “a” on the left illustrates an effect of the ashing process using O2. The diagram labeled “b” on the right illustrates an effect of the ashing process using a H2/N2 gas mixture. In the photo resist stripping process after an LDD implantation, the O2 rich ashing does not generate residues, however, the result of Si recess is far from satisfactory. The total Si recess in an LDD region is larger than 15 Å. The H2/N2 gas mixture rich ashing does not generate residues either, and generates less Si recess (8 Å less than the O2 rich ashing). But the reaction between Si- and H-results in a severe loss of silicon nitride offset spacer; in some cases, the silicon nitride offset spacer is even removed, as shown in the circle on diagram “b” of FIG. 1.
FIG. 2 shows the impact of O2 to the H2/N2 gas mixture ratio on the offset spacer critical dimension of PMOS and NMOS devices. As shown in FIG. 2, the higher the ratio of O2 to the H2/N2 gas mixture is, the smaller a reduction on the critical dimension of the offset spacer of PMOS and NMOS becomes.